Thermal printer for a low cost electronic mail terminal

ABSTRACT

An electronic mail terminal 10 that includes a keyboard 12, multi-character display 14, thermal printer 22, tape drive 118, and communication and control circuitry. The tape drive and printer mount to molded-in features of a base housing 20. A printed circuit board 28 containing the communication and control circuitry and the display is mounted to molded-in features of the base housing. The keyboard plugs into the printed circuit board from above and is supported at the sides by stand-offs in the base housing. A cover housing 88 mates with molded-in features of the base housing. The tape drive mechanism includes cassette guides 98 and 376 that are molded into the base and cover housings and a spring loaded motor mount that automatically compensates for wear and positioning tolerances. A printer carriage motor 184 drives a toothed belt 182 to position a carriage 138 which holds a thermal print head 140. An optical encoder 146 and a mechanical switch 194 provide position feedback of carriage position to the control circuitry. A paper advance motor 256 advances the paper through a worm gear train 256, 262 with position feedback provided by a mechanical switch 280. The printer carriage is a molded plastic runner to which the toothed belt and the thermal print head attaches. A metal clip 340 provides an easily removable mounting for the print head.

This is a division of application Ser. No. 06/465,040, filed Feb. 8,1983, and issued June 18, 1985 as U.S. Pat. No. 4,524,242.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to printing devices suitable foruse as electronic mail terminals and more particularly to a thermalprinter, which may be inexpensively packaged in a single housing of anelectronic mail terminal.

2. Description of the Prior Art

Electronic mail involves the communication of printed messages frompoint to point using telephone lines. Advantages of electronic mail overour existing postal service include delivery speed and reliability. Asthe name specifies, electronic mail would comprise an electronictransmission facility and a printed record. Although there are many waysto provide the printed record, economic realities dictate the use of aninexpensive technique for providing such a record. Telex and TWXcommunication services are examples of currently available forms ofelectronic mail, which provide a printed copy. However, these servicesdo not provide the same service and flexibility as electronic mail.

SUMMARY OF THE PRESENT INVENTION

It is an object of this invention to provide a printer which can beincorporated in the base housing of an inexpensive, small electronicmail terminal.

It is another object of the invention to provide a printer for printingcharacters on thermally sensitive paper.

Another object of the present invention is to provide a low cost printerusing direct current motors with position feedback sensors forpositioning the print head and for advancing the paper.

Still another object of the present invention is to provide a thermalprinter with an easily replaceable print head.

Still another object of the present invention is to provide a print headpositioning mechanism having friction drives that compensate for wear.

Briefly, the printer includes one direct current electric motor toposition the printer carriage and another to advance the paper. Theprinter carriage motor drives a toothed belt to laterally position athermal print head. An optical encoder and a mechanical switch provideposition feedback to the control circuitry. The paper advance motoradvances the paper through a worm gear train with position feedbackprovided by another mechanical switch.

The printer carriage is a molded plastic runner to which the toothedbelt and the thermal print head attaches. A metal clip provides amounting for the print head which permits easy removal for replacement.Printing speed and reliability are enhanced by the low mass of theprinter carriage.

IN THE DRAWING

FIG. 1 is an isometric view of an electronic mail terminal according tothe present invention;

FIG. 2 is an exploded view in isometric perspective of the majorsubassemblies of the terminal of FIG. 1;

FIG. 3 is a sectional view of the terminal of FIG. 1 as taken along thesection line shown in FIG. 1;

FIG. 4 is a rear elevation view of the terminal of FIG. 1;

FIG. 5 is an exploded view in isometric perspective of the basesubassembly of the terminal of FIG. 1. A printer mechanism and a tapedrive mechanism are shown;

FIG. 6 is an exploded view in isometric perspective of the left sideplate and printer head positioning mechanism of the printer mechanism ofFIG. 5;

FIG. 7 is a sectional view of the printer head positioning mechanism ofFIG. 6 as taken along the section line shown in FIG. 6;

FIG. 8 is an exploded view in isometric perspective of the right sideplate and paper advance mechanism of the printer mechanism of FIG. 5.Note that the perspective viewpoint of FIG. 8 is from the upper rightcorner of the terminal of FIG. 1, while the perspective viewpoint of allother isometric views is from the upper left corner of the terminal;

FIG. 9 is a sectional view of a portion of the paper advance mechanismas taken along the section line shown in FIG. 8;

FIG. 10 is a side elevation view of a portion of the right side plate ofthe printer mechanism;

FIG. 11 is an exploded view in isometric perspective of a print headmechanism utilized in the printer mechanism of FIG. 5;

FIG. 12 is a sectional view of the print head mechanism as taken alongthe section line shown in FIG. 11;

FIG. 13 is an exploded view in isometric perspective of the tape drivemechanism of FIG. 5;

FIG. 14 is a side elevation sectional view of the tape drive mechanismas taken along the section line shown in FIG. 13;

FIG. 15 is a front elevation sectional view of the tape drive mechanismas taken along the section line shown in FIG. 14;

FIG. 16 is a functional block diagram of the computer and controlcircuits of the terminal of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1 of the drawing, the preferred embodiment of anelectronic mail terminal, according to the present invention, isillustrated at 10. A keyboard 12 is provided for the manual entry ofoutgoing messages. Both outgoing and incoming messages may be displayedat a multi-character display 14 or printed onto paper 16. A tapecassette 18 serves as a non-volatile memory for storage and filing ofmessages when inserted into a tape drive located within the terminal.

In the interest of minimizing manufacturing cost, a base housing 20 hasmolded-in features for the attachment of all sub-assemblies of theterminal 10, see FIG. 2. A thermal printer 22 is affixed to the leftside of base housing 20 by a hook 24. As will be later described, theright side of the printer is affixed to the base housing in a similarmanner.

The front edge 26 of a circuit board 28, which contains thecommunication, computer, and control circuitry, is positioned under tabs30, 32 and 34 located across the front edge of the base housing. Twostand-offs 36 and 38, located along the sides of the base housing,support the sides of the circuit board. Pins 40 in the top of thestand-offs fit into corresponding holes 42 and 44 in the circuit boardto provide lateral positioning for the circuit board. The circuit boardis oriented with electronic components positioned on the bottom side.The multicharacter display 14 is supported by and is electricallycoupled to the circuit board by a connector 46. Electrical connectionsfrom the circuit board to the printer 22 and to the tape drive areprovided by cables which will be described below.

The terminal 10 is powered by an external power supply 48 and connectsto a telephone network via a communication cable 50. Communication cable50 has a connector 52 at one end which plugs into the circuit board 28through an access hole 54, and a telephone type connector 56 at theother end which plugs into a telephone receptacle for connection to thetelephone network. The external power supply 48 converts standardalternating current, which is input to the power supply via a standardelectrical plug 58 and a cable 60, into low voltage direct current forpowering the circuit board and other subassemblies. Direct current isinput to the circuit board via a cable 62 and a connector 64 which plugsinto the circuit board through an access hole 66. Advantages to usingthe external power supply rather than an internal power supply includereduced terminal size and reduced heat generation within the terminal.

To provide a compactly sized terminal 10, the keyboard 12 is positioneddirectly above the circuit board 28. A thin insulative sheet 68 servesto insulate the switches of the keyboard from the circuitry of thecircuit board. The keyboard includes a base 70 of substantial rigiditywith tabs 72, 74, 76 and 78 protruding from the sides. These tabs matewith stand-offs 80, 82, 84 and 86, respectively, which are molded intothe base housing 20 to position and support the keyboard.

A cover housing 88 completes the terminal assembly by enclosing thesubassemblies within. A keyboard opening 90 provides access to the keysof the keyboard 12. Visual access to the display 14 is gained through adisplay opening 92 located at the rear face of a depression 94. Accessto the tape drive is through a cassette opening 96 which includes aright cassette guide 98 to aid in positioning the tape cassette 18.Paper 16 from the printer 22 is routed beneath a transparent bail 100,which includes teeth 102 for tearing the paper for removal.

FIG. 3 illustrates the means for attaching the cover housing 88 to thebase housing 20. Two front hooks 104 protrude downward from the front ofthe cover housing and mate with two front slots 106 and 108 in the frontof the base housing. At the rear of the terminal 10, two rear hooks 110and 112 protrude upward from the rear wall of the base housing. Rearhooks 110 and 112 mate with corresponding rear slots 114 and 116 in therear wall of the cover housing to lock the rear portion of the housingsand to pull the cover housing rearward to lock the front hooks into thefront slots.

FIG. 3 also illustrates the relative positioning of the circuit board28, keyboard 12, printer 22, and tape drive 118 within the terminal 10.Stand-off 38 and tab 32 incline the circuit board relative to the basehousing. As a result, the larger sized electronic components arepositioned toward the rear of the circuit board. The keyboardelectrically connects to the circuit board through a connector 120.Stand-offs 80 and 82 incline the keyboard to the same degree as thecircuit board. The means for mounting the printer and the tape drivewill be described below.

The rear of the terminal 10 provides a space for a roll of paper 122 foruse with the printer 22. FIG. 4 illustrates the access that is providedfor the installation of the roll of paper. The rear walls of the coverand base housings 88 and 20 are relieved to permit the roll of paper tobe installed on two nipples 124 and 126 that are part of the printer.

FIG. 5 illustrates the installation of the printer 22 and the tape drive118 onto the base housing 20. The printer includes a left side plate 128and a right side plate 130 with two shafts 132 and 134 and a platen 136disposed therebetween. A carriage 138 with a print head 140 attachedthereto is positioned along the shafts by carriage positioning means 142that is mounted to the left side plate. Paper is advanced through theprinter by paper advance means 144 that is mounted to the right sideplate.

The printer 22 is assembled as a subassembly as shown in FIG. 5 beforeinstallation onto the base housing 20. A carriage position sensor 146 isfirst mounted onto posts 148 and 150 that protrude up from the basehousing. Then the remainder of the printer is lowered into position.Hooks 24, 152, and 154 position and retain plates 128 and 130 bylatching over flats 156, 158, and 160 (shown in FIG. 8).

The tape drive 118 is assembled by installing its various componentsonto the base housing 20, rather than forming a separate subassembly asthe printer 22. A tape drive motor 162 is installed by inserting it intoa motor mounting bracket 164. Similarly, a read/write head 166, a tapesensor 168, and a cassette retention spring 170 are installed byinserting them into their respective mounting brackets 172, 174, and176.

Turning now to FIGS. 6 and 7, the carriage advance means 142 of theprinter 22 will be described. A drive pulley 178 and an idler pulley 180(FIG. 8) are rotatably attached to plates 128 and 130, respectively. Atoothed belt 182 is looped around both pulleys and is affixed to thecarriage 138 to couple the carriage to the drive pulley. Thedisplacement of the carriage is thus coupled to rotation of the drivepulley. A carriage drive motor 184 having a drive roller 186 affixed toits shaft 188 provides the means for rotating the drive pulley. Drivepulley 178 is divided into a belt pulley 190 that serves as a pulley forthe toothed belt and a drive wheel 192 of a larger diameter thatcontacts the drive roller.

The position of the carriage 138 is known to the control circuitry ofthe circuit board through the carriage position sensor 146 and anend-of-line switch 194. The drive wheel portion of the drive pulley hasa circumferential lip with equally spaced radial slots. Rotation of thedrive pulley is sensed by the carriage position sensor 146. Sensor 146utilizes a light source 196 and a light detector 198 positioned onopposite sides of the lip to detect movement of the slots and, hence,rotation of the drive pulley. This arrangement provides carriageposition feedback to the control circuitry, and as a result, aninexpensive direct current motor can be used rather than a more costlystepper motor while maintaining acceptable carriage positioningaccuracy. The end-of-line switch is activated by the carriage when it ispositioned at the far left side of the printer. This provides areference point from which the position of the carriage is computed bythe control circuitry according to the drive pulley rotation detected bysensor 146. The carriage drive motor 184, the carriage position sensor146, and the end-of-line switch 194 are respectively connected to thecontrol circuitry on the circuit board by cables 200, 202, and 204.

The left side plate 128 provides means for mounting the carriagepositioning means 142 described above. Plate 128 includes two brackets206 and 208 for mounting motor 184. Brackets 206 and 208 projecthorizontally from the left side of plate 128 and are spaced apart by adistance equal to the length of the motor. The brackets have openings210 and 212 that are sized to accept flanges 214 of the motor. Toinstall the motor, the ends of the brackets are first deflected apart.Then the shaft end of the motor is inserted into the lower opening 212and flange 214 is inserted into the upper opening 210. When the bracketsare released, they return to their horizontal positions and the motorinstallation is complete.

The left side plate 128 provides means for mounting the drive pulley 178so that any wear in the drive roller 186 or the drive wheel 192 isautomatically compensated for to ensure proper contact therebetween. Adowel pin 216 inserted into an axial hole 217 in the drive pulleyprovides an axis for the drive pulley to rotate. Mounting means for thedowel pin and the drive pulley includes upper, central, and lower pulleybrackets 218, 220, and 222, which project horizontally from the leftside of the plate. The lower pulley bracket 222 has a slotted recess 224open to the top and left side with a bar 226 disposed across the sideopening. The slotted recess is dimensioned so that the dowel pin can beinserted from the top and can move toward and away from the motor 184,but can not move sideways. The central bracket 220 is positioned abovethe lower bracket by a distance equal to the height of the drive pulley.A slot 228 in the central bracket is positioned above the slotted recessand is open to the side. The width of slot 228 is equal to the diameterof the dowel pin. The upper bracket 218 is positioned above both theslotted recess and the slot.

To install the drive pulley, it is first placed between the central andlower brackets 220 and 222. A coil spring 230 is then inserted in theslotted recess to bias the bottom of the dowel pin toward the motor 184,thus preloading the drive wheel 192 toward the drive roller 186. Aseither the drive wheel or the drive roller wears, spring 230 acts tomove the dowel pin to compensate for the wear.

Next, the upper bracket is deflected and the dowel pin 216 is insertedthrough the slot 228, the axial hole 217 of the drive pulley, and intothe slotted recess 224 of the lower bracket. At this point, the top ofthe dowel pin is level with the underside of the upper bracket. When theupper bracket is released, it returns to its undeflected position, thusretaining the top of the dowel pin. The orientation of the axis orrotation of the drive pulley and the motor are illustrated as beingvertical, but other orientations that are perpendicular to thehorizontally oriented shafts 132 and 134 would work equally as well.

Moving our attention to the right side of the left side plate 128, meansfor mounting the shafts 132 and 134, the platen 136, and the end-of-lineswitch 194 will now be described. Mounting for the switch is provided bytwo posts 232 and 234 to which the switch is adhesively bonded. Shaft132 has a roller 236 affixed at the end thereof with a short length ofthe shaft protruding. Platen 136 has a dowel pin 238 inserted at theleft end thereof which provides an axle upon which another roller 240 isfree to rotate. The end of dowel pin 238, the protruding end of shaft132, and the end of shaft 134 are mounted into holes (not shown) in theright side plate 128, permitting rollers 236 and 240 and shaft 132 tofreely rotate.

The paper advance means 144 and the right side plate 130 of the printer22 are illustrated in detail in FIGS. 8, 9 and 10. The shafts 132 and134 and the platen 136 are mounted to the right side plate in a fashionsimilar to the left side. A roller 242 is affixed to the end of shaft132 like roller 236 on the left side. Platen 136 has a dowel pin 244inserted into the center 246 thereof, as on the left side, and a roller248 is free to rotate on the dowel pin. Shafts 132 and 134 and platten136 are mounted to plate 130 by insertion into holes 250, 252, and 254,respectively. Shaft 132 protrudes through hole 250 for coupling to thepaper advance means.

Paper 16 is advanced through the printer 22 by causing shaft 132 torotate. Paper from the roll of paper 122 is grasped along each edge byrollers 236 and 240 on the left and rollers 242 and 248 on the right.Since rollers 236 and 242 are affixed to shaft 132 and rollers 242 and248 are free to rotate about their respective dowel pins 238 and 244,rotation of shaft 132 in a counterclockwise direction, as viewed in FIG.8, advances the paper.

Two means for rotating shaft 132 are provided, one manual and oneautomatic. The automatic means employs a line feed motor 256 with a worm258 affixed to its shaft 260 to drive a worm gear 262. The worm gear iscoupled to shaft 132 through a thumb wheel 264 which is affixed to theend of shaft 132. The manual means employs only the thumb wheel torotate shaft 132.

In order to couple the worm gear 262 to the thumb wheel 264 duringautomatic paper advance and to decouple them during manual paperadvance, the worm gear operates as a clutch. The worm gear has radialteeth 266 equally spaced about its right face which mate withcorresponding radial teeth 268 on the left face of the thumb wheel. Thefaces of the teeth are inclined so as to couple the thumb wheel to theworm gear when the worm gear rotates counterclockwise, while permittingthe thumb wheel to rotate counterclockwise when the worm gear isstationary. When an automatic paper advance is desired, the motor 256turns the worm 258 which rotates the worm gear. Axial faces 270 of theteeth 266 and 268 lock the thumb wheel to the worm gear so that bothrotate counterclockwise, advancing the paper. However, when a manualpaper advance is desired, inclined faces 272 of the teeth cause the wormgear to move left, compressing a spring 274. Spring 274 is disposedaround shaft 132 within a cavity 276 in the worm gear and acts to biasthe worm gear toward the thumb wheel. As the worm gear moves left, thethumb wheel is permitted to rotate counterclockwise. After the thumbwheel and shaft 132 has rotated an amount corresponding to the pitch ofthe teeth, the spring moves the worm gear to the right to its startingposition, ready for either automatic or manual paper advance.

A feedback mechanism provides position information concerning the wormgear 262 to the control circuitry on the circuit board 28. The left sideof the worm gear has notches 278 equally spaced about its periphery. Thepitch between the notches corresponds to the degree of rotation of shaft132 that is equivalent to advancing the paper 16 by one line. A linefeed switch 280 is electrically coupled to the control circuitry viacable 282 and includes a follower 284 that follows the contours of theleft side of the worm gear. The contours of the notches are such thatthe follower readily may follow without binding. When the worm gear ispositioned such that the follower is in a notch, switch 280 is closed.When the worm gear is positioned such that the follower is not in anotch, the switch is open. In this manner, the control circuit canprovide power to the line feed motor 256 through its cable 286 until theswitch indicates that a line feed has been completed.

The right side plate 130 provides means for mounting the paper advancemeans 144 described above. Plate 130 includes two brackets 288 and 290for mounting the line feed motor 256. Brackets 288 and 290 project fromthe right side of plate 130 and are spaced apart by a distance equal tothe length of the motor 256. The brackets have openings 292 and 294 thatare sized to accept flanges 296 of the motor 256. To install the motor,the ends of the brackets are first deflected apart. Then, the shaft endof the motor is inserted into the forward opening 292 and the rear endof the motor is inserted into the rear opening 294. When the bracketsare released, they return to their undeflected positions and the motorinstallation is complete. The line feed switch 280 is mounted to theplate 130 by adhesively bonding it to two posts 298 and 300. Posts 298and 300 are formed in a projection 302 out of the upper rear portion ofthe right side plate.

The idler pulley 180 is mounted to the right side plate 130 by a dowelpin 304 that is locked in place by shaft 134. Plate 130 includes upperand lower grooves 306 and 308, a shelf 310 with a horizontal slot 312that is open to the left, and a clearance opening 314. To install theidler pulley, it is first placed on the shelf with its center hole 316positioned over the slot. Next, the dowel pin is inserted into thecenter hole from above and is lowered through the slot until the bottomof the dowel pin reaches the bottom of the lower groove. At that point,the pulley is mounted with the upper and lower grooves counteracting anytensile force in the toothed belt 182. To finish the installation, shaft134 is inserted into hole 252 to prevent the dowel pin from lifting.

The right side plate 130 is affixed to the base housing 20 by hooks 152and 154 latched over flats 158 and 160, as described above. Flats 158and 160 are horizontal surfaces in arms 318 and 320 which projecthorizontally to the right from plate 130. The arms act to give the rightside plate a stable base for attachment to the base housing.

The carriage 138 and the print head 140 are illustrated in detail inFIGS. 11 and 12. The carriage has a left runner 322 and a right runner324 which are both connected to a central body 326. The left and rightrunners 322 and 324 have contact surfaces 326 and 328 that encircleshaft 134 and couple the carriage thereto. Contact surfaces 326 and 328are formed from connected concave surfaces that are alternately openabove and below the carriage. This type of construction permits thecarriage to be readily fabricated by a molding process. The left andright runners respectively have left and right slots 330 and 332 intowhich the ends of the toothed belt 182 are inserted. The toothed beltcouples the carriage to the carriage positioning means 142. A finger 334that protrudes to the left from the left runner actuates the end-of-lineswitch 194 when the carriage is positioned at the far left. The centralbody has a hook 336 formed therein that contacts shaft 132 to limit therotation of the carriage about shaft 134.

The print head 140 is removably coupled to the central body 326 of thecarriage and is operable for locally heating the thermal paper 16 totransfer characters thereto. Print head 140 includes an element 338 thatis adhesively bonded to a clip 340 and is electrically coupled to thecontrol circuitry on the circuit board 28 via a cable 342. Clip 340 ispreferably fabricated from a thin strip of metal formed to a vee shape.The element is bonded to the rear facing leg of the vee. Mountingfeatures are formed into the front facing leg which include tabs 344 and346 that open to the top and face forward and a tab 348 that opens tothe bottom and faces rearward. In addition, the bottom edge 350 of thefront facing leg is angled forward.

To install the print head 140 into the carriage 138, the bottom edge 350of the clip 340 is positioned above the carriage, the two legs of theclip are squeezed together, and then inserted into the groove formed byfaces 352 and 354 of the central portion 326. As the clip is lowered,edge 350 deflects forward off of an internal shoulder 356. When fullyinserted, tab 348 contacts shoulder 346 and tabs 344 and 346 lock theclip in position against another internal shoulder 358. In the installedposition, the clip biases the element 338 toward the platten 136 toprovide intimate with the paper 16 for good printing results. To removethe print head, the clip is squeezed and rotated rearward to releasetabs 344 and 346 from shoulder 358, then lifted.

The installation and function of the tape drive 118 is shown in detailin FIGS. 13, 14 and 15. The tape drive consists of a tape drive motor162, a tape head 166, an end-of-tape sensor 168 and cassette positioningmeans. Motor 162, head 166, and sensor 168 are mounted to variousfeatures that are molded into the base housing 20. This tape drive isadapted for use with tape cassettes 18 for having a tape drive wheel 360and an internal band for driving the tape reels, grooves 362 and 364along two sides for positioning the cassette, a door 366 for protectingthe magnetic tape 368, and a mirror 370 located behind the magnetic tapeto aid in sensing the transparent leader tape at either end of themagnetic tape.

Cassette positioning is provided by various features of the base housing20 and cover housing 88. A left support 37 molded into the base housingprovides an insertion stop 374 and a left guide rail 376. The left guiderail acts to position the left groove 362 of the cassette 18 as well asopen the door 366 when the cassette is inserted. The insertion stop 374acts to limit the downward insertion of the cassette on the left side.The right cassette guide 98, attached to the cover housing positions theright groove 364 of the cassette. Downward insertion of the right sideof the cassette is limited by a notch 378 in a right support 380. Thecassette is biased toward the left guide rail and the notch 378 in aright support 380. The cassette is biased toward the left guide rail andthe notch 378 by the cassette retention spring 170 which acts on a notch382 in the cassette. Cassette retention spring 170 is preferablyfabricated from a metal strip and is installed by sliding a foot portion384 into an opening 386 in the right support 380. The cassette retentionspring has an inclined surface 388 at the upper end thereof which isdeflected by the cassette when inserted.

The tape drive motor 162 is electrically coupled to the controlcircuitry by a cable 390 and is operable for rotating the tape drivewheel 360 through a drive roller 392 affixed to the motor shaft 394.Motor 162 is pivotably mounted to the base housing 20 and biased towardthe cassette 18 so as to automatically compensate for wear on driveroller 392 and for tolerances in the position of the cassette. The motoris retained by the motor mounting bracket 164 which consists of an arch396 having a radius equal to that of the motor and two legs 398 and 400that span the motor. A front flange 402 of the motor is similarlyretained by another bracket 404 located rearward of bracket 164. A tab406 affixed to the front of the motor is retained by a notch 408 in amotor support 410 to prevent motor rotation. The motor support isoperable for recessing into the base housing to permit the motor to beinstalled into brackets 164 and 404. A rotary coil spring 412 is mountedto the base housing on a spring mount 414. One arm 416 of the spring 412is positioned under flange 402 to provide the biasing force to preloadthe drive roller 392 against the tape drive wheel 360. When the cassetteis withdrawn, the motor shaft 394 pivots upward until the flangecontacts the bracket 404.

The tape head 166 is provided to magnetically record messages onto themagnetic tape 368 of the cassette 18 and to playback previously recordedmessages. Two tabs 418 and 420 on the tape head slip into slots in twostand-offs 422 and 424 to provide a mounting for the tape head. A cable426 electrically couples the tape head to the control circuitry.

The control circuitry must know when the end of the magnetic tape 368 isreached in a winding operation to prevent damage to the cassette 18. Toaccomplish this, a light source 428 and an aperture 430 are providedbelow and adjacent to the mirror 370 in the cassette and the lightsensitive tape sensor 168 is positioned level with and adjacent to themirror. The opaque magnetic tape prevents light from the light sourcefrom reaching the sensor when the magnetic tape is proximate the tapehead 166. However, the transparent leader tape permits light from thelight source to reach the mirror and reflect to the sensor, therebyindicating that the magnetic tape has been fully wound on a reel. Thesensor and the light source are coupled to the control circuitry bycables 432 and 434.

FIG. 16 is a block diagram that depicts the functional operation of theelectronic mail terminal 10 according to the preferred embodiment of thepresent invention. The bulk of the circuitry is contained on the circuitboard 28. The operation of those components not located on the circuitboard is described above.

The brain of the circuitry is a microprocessor 436 which functions asboth a computer and as a controller. The circuitry is bus oriented witha control bus 438, a data bus 440, and an address bus 442. Themicroprocessor interfaces to the address bus through a buffer 444,interfaces to the control bus both directly and through a state decoder446, and interfaces directly to the data bus. A programmable read onlymemory (PROM) 448 provides program storage, and a random access memory(RAM) 450 provides temporary storage of messages. Both PROM and the RAMare connected to the address bus and the data bus. The microprocessoraccesses the memory through a memory decoder 452 that is connected tothe control and address buses.

Messages are input to the terminal through the keyboard 12 which sendscharacters to the data bus 440 through a buffer 454. The keyboard isalso connected to a decoder/driver 456 for displaying characters on themulti-character display as they are typed on the keyboard. Charactersfrom the data bus through a buffer 458 are also input to thedecoder/driver for display. Input power 460 for the display is routedthrough a driver 462 that is connected to the address bus 442 to permitthe selection of a variety of display modes.

Messages to be printed are transmitted to a print head driver 464 overthe address bus 442. The carriage sensor 146 and the end-of-line switch194 are connected in series to the control bus 438 and act to disablethe write enable circuit 466 if the carriage is not properly positioned.The print head 140 requires both an enable signal from the write enablecircuit and a print signal from the print head driver before it prints.During the printing routine, a carriage motor driver 468 which isconnected to the control bus directs the carriage motor 184 to properlyposition the carriage 138 for printing. When a line feed is required, aline feed motor driver circuit 470 powers the line feed motor 256 untilthe line feed switch 280 indicates completion.

Messages to be kept are stored on magnetic tape by the tape drive. Whena message is to be stored, a tape motor driver 472, which is directed bythe control bus 438, powers the tape drive motor 162 until the tape isproperly positioned. Then a tape write latch 474, which is connected tothe control bus, enables a tape write driver 476 to transfer data fromthe data bus 440 to the tape head 166 for recording. Data to be readfrom the magnetic tape is read by the tape head and transferred througha tape read buffer 478 to the data bus. The end-of-tape sensor 168 isconnected to the control bus to prevent the tape drive motor from overwinding the cassette.

All outgoing messages are output to the telephone cable 50 through amodem 480 of conventional design. Control signals from the control bus438 determine destination and transmission rate, while the actual datato be transmitted comes from the data bus 440. Incoming messages enterthe terminal 10 through the modem and the data bus.

Although the present invention has been described above in terms of apreferred embodiment specifically designed as an electronic mailterminal, it will be understood that the system or various parts thereofcould be adapted for other applications. Accordingly, it is applicants'intent that the appended claims be interpreted as covering all suchalterations, modifications or other applications as fall within the truescope of the invention.

What is claimed is:
 1. A printer for printing characters onto thermallysensitive paper, said printer comprising:means forming a base having aplanar surface; a first plate coupled to said base and disposedsubstantially perpendicular to said surface of said base; a second platecoupled to said base and disposed laterally spaced apart from andapproximately parallel to said first plate; a first shaft disposedbetween and rotatably coupled to said first and second plates, saidfirst shaft being disposed approximately parallel to said surface ofsaid base and carrying at least one paper engaging roller; a secondshaft disposed between and coupled to said first and second plates anddisposed parallel to said first shaft; means forming a platen disposedbetween and coupled to said first and second plates and disposedparallel to said shafts; means forming a passageway between said platenand said first and second shafts for receiving a continuous web ofpaper; a carriage slideably coupled to said first and second shafts;print head means removeably coupled to said carriage and operable forlocally heating said paper to transfer a character pattern thereto;carriage positioning means coupled to said plates and to said carriagefor laterally positioning said carriage along said shafts, said carriagepositioning means including;a first pulley coupled to said first plateand rotatable about an axis perpendicular to said shafts, said firstpulley having an upper portion and a lower portion; a second pulleycoupled to said second plate and rotatable about an axis parallel tosaid axis of said first pulley, said second pulley being coupled to saidsecond plate by a dowel pin disposed at the axis of said second pulley,said dowel pin being retained in a groove in said second plate by saidsecond shaft; belt means for translating said carriage in response torotation of said first pulley, said belt means forming a loop aroundsaid upper portion of said first pulley and said second pulley with bothends thereof being affixed to said carriage; and belt drive meanscoupled to said lower portion of said first pulley for rotating saidfirst pulley.
 2. A printer as recited in claim 1 wherein said belt meansis a toothed belt with the teeth thereof disposed toward the inside ofsaid loop, and wherein said upper portion of said first pulley hasexternal teeth operable for mating with the teeth of said toothed belt.3. A printer as recited in claim 1 wherein said carriage positioningmeans further includes a carriage position switch coupled to said firstplate and operable for sensing the presence of said carriage when saidcarriage is proximate said first plate.
 4. A printer as recited in claim1 wherein said carriage positioning means further includes servo meansfor sensing the rotation of said first pulley.
 5. A printer as recitedin claim 4 wherein said servo means is a photodetector sensor with alight source and detector and an encoder affixed to said first pulley,said sensor senses the rotation of said encoder and hence the rotationof said first pulley.
 6. A printer as recited in claim 1 wherein saidbelt drive means is a first rotary electric motor having a shaft with adrive roller affixed thereto, said first motor being coupled to saidfirst plate such that the periphery of said drive roller is in rollingcontact with the periphery of said lower portion of said first pulley,said first motor being operable for rotating said first pulley.
 7. Aprinter as recited in claim 1 wherein said first and second plates areremovably affixed to the surface of said base.
 8. A pritner as recitedin claim 1 wherein said paper is a cylindrical roll of paper having anaxial passage therethrough and said first and second plates includemeans for removeably mounting said roll of paper comprising a protrusionon each of said plates for insertion into said axial passage to retainsaid paper.
 9. A printer as recited in claim 1 wherein said shafts andsaid platen are coupled to said first and second plates by perforationsin said first and second plates.
 10. A printer as recited in claim 1wherein said paper advance means includes:a second rotary electric motormounted to said second plate; gear means, coupled to said second motorand to said first shaft, for transforming rotation of said second motorinto rotation of said first shaft; second sensing means mounted to saidsecond plate for sensing the rotation of said gear means; manual paperadvance means for advancing said paper without operating said secondmotor; two first rollers, affixed to the ends of said first shaft forrotation therewith and disposed proximate said plates; and two secondrollers, rotatably coupled to said platen and disposed proximate saidfirst rollers, said first rollers acting to advance said paper wheneversaid first shaft is rotated.
 11. A printer as recited in claim 10wherein said gear means is a worm affixed to the drive shaft of saidsecond motor and a worm gear coupled to said first shaft.
 12. A printeras recited in claim 11 wherein said worm gear has notches equally spacedabout the periphery thereof and wherein said second sensing means is anelectromechanical switch affixed at one end to said second plate andoperable for sensing rotation of said worm gear according to thecircumferential position of said notches so as to indicate the degree ofpaper advance.
 13. A printer as recited in claim 11 wherein said manualpaper advance means has a thumb wheel affixed to the end of said firstshaft and said worm gear is releasably coupled to said thumb wheel byinterlocking teeth and is biased toward said thumb wheel by a spring,said spring and said interlocking teeth permit said worm gear to camaway from said thumb wheel to rotate said first shaft by said thumbwheel, said spring and said interlocking teeth lock said thumb wheel tosaid worm gear when said worm gear is rotated by said worm.
 14. Aprinter for printing characters onto thermally sensitive paper, saidprinter comprising:means forming a base having a planar surface; a firstplate coupled to said base and disposed substantially perpendicular tosaid surface of said base, said first plate including: upper and lowermotor brackets affixed to said first plate for mounting said first motorthereon, said upper and lower motor brackets being deflectable toaccommodate installation of said first motor; and upper, central, andlower pulley brackets affixed to said first plate for mounting saidfirst pulley thereon, said lower pulley bracket having a recess thereinthat is open to the top, said central pulley bracket having a slottherein disposed above said recess, and said upper pulley bracket beingdisposed above said slot and recess and deflectable to a position notabove said slot and recess; a second plate coupled to said base anddisposed laterally spaced apart from and approximately parallel to saidfirst plate; a first shaft disposed between and rotatably coupled tosaid first and second plates, said first shaft being disposedapproximately parallel to said surface of said base and carrying atleast one paper engaging roller; a second shaft disposed between andcoupled to said first and second plates and disposed parallel to saidfirst shaft; means forming a platen disposed between and coupled to saidfirst and second plates and disposed parallel to said shafts; meansforming a passageway between said platen and said first and secondshafts for receiving a continuous web of paper; a carriage slideablycoupled to said first and second shafts; print head means removeablycoupled to said carriage and operable for locally heating said paper totransfer a character pattern thereto; carriage positioning means coupledto said plates and to said carriage for laterally positioning saidcarriage along said shafts, said belt drive means including a firstrotary electric motor having a shaft with a drive roller affixedthereto, said first motor being coupled to said first plate such thatthe periphery of said drive roller is in rolling contact with theperiphery of said lower portion of said first pulley, said first motorbeing operable for rotating said first pulley;a first pulley coupled tosaid first plate and rotatable about an axis perpendicular to saidshafts, said first pulley having an upper portion and a lower portion; asecond pulley coupled to said second plate and rotatable about an axisparallel to said axis of said first pulley, said second pulley beingcoupled to said second plate by a dowel pin disposed at the axis of saidsecond pulley, said dowel pin being retained in a groove in said secondplate by said secod shaft; belt means for translating said carriage inresponse to rotation of said first pulley, said belt means forming aloop around said upper portion of said first pulley and said secondpulley with both ends thereof being affixed to said carriage; and beltdrive means coupled to said lower portion of said first pulley forrotating said first pulley.
 15. A printer as recited in claim 14 andfurther comprising a spring installed in said recess and operable tobias the lower portion of said dowel pin toward said first motor,thereby biasing said lower portion of said first pulley toward saiddrive roller.
 16. A printer as recited in claim 14 wherein said firstplate is fabricated by a molding process and said motor brackets andsaid pulley brackets are integral portions of said first plate.
 17. Aprinter for printing characters onto thermally sensitive paper, saidprinter comprising:means forming a base having a planar surface; a firstplate coupled to said base and disposed substantially perpendicular tosaid surface of said base; a second plate coupled to said base anddisposed laterally spaced apart from and approximately parallel to saidfirst plate, said second plate including two motor brackets affixedthereto for mounting said second motor thereon, said motor bracketsbeing deflectable to permit the installation of said second motor; afirst shaft disposed between and rotatably coupled to said first andsecond plates, said first shaft being disposed approximately parallel tosaid surface of said base and carrying at least one paper engagingroller; a second shaft disposed between and coupled to said first andsecond plates and disposed parallel to said first shaft; means forming aplaten disposed between and coupled to said first and second plates anddisposed parallel to said shafts; means forming a passageway betweensaid platen and said first and second shafts for receiving a continuousweb of paper; a carriage slideably coupled to said first and secondshafts; print head means removeably coupled to said carriage andoperable for locally heating said paper to transfer a character patternthereto; carriage positioning means coupled to said plates and to saidcarriage for laterally positioning said carriage along said shafts, saidcarriage positioning means including,a first pulley coupled to saidfirst plate and rotatable about an axis perpendicular to said shafts,said first pulley having an upper portion and a lower portion; a secondpulley coupled to said second plate and rotatable about an axis parallelto said axis of said first pulley, said second pulley being coupled tosaid second plate by a dowel pin disposed at the axis of said secondpulley, said dowel pin being retained in a groove in said second plateby said second shaft; belt means for translating said carriage inresponse to rotation of said first pulley, said belt means forming aloop around said upper portion of said first pulley and said secondpulley with both ends thereof being affixed to said carriage; and beltdrive means coupled to said lower portion of said first pulley forrotating said first pulley; and paper advance means coupled to saidsecond plate and to said first shaft for rotating said first shaftthereby causing said roller to advance said paper through said printer.18. A printer as recited in claim 17 wherein said carriage includes:atleast one perforation therethrough to guide said carriage along saidsecond shaft; hook means disposed proximate said first shaft forrestricting the rotation of said carriage about said second shaft; beltattachment means for attaching said belt means to said carriage; andhead mounting means for removably coupling said print head to saidcarriage.
 19. A printer as recited in claim 18 wherein said print headincludes:a printing element capable of generating heat; and a metal clipadhesively bonded to said printing element, said metal clip forminggenerally an inverted vee shape and operable for biasing said printingelement toward said platen and said paper.
 20. A printer as recited inclaim 19 wherein said head mounting means is one or more shoulders insaid carriage and said metal clip has one or more tabs formed in thesurface thereof and disposed to cooperate with said shoulders toremovably couple said print head to said carriage.